1. adj. As yet unexplained, or
too complicated to explain; compare automagically
and (Arthur C.) Clarke's Third Law: “Any sufficiently advanced
technology is indistinguishable from magic.” “TTY echoing is
controlled by a large number of magic bits.” “This routine
magically computes the parity of an 8-bit byte in three
instructions.”

2. adj. Characteristic of
something that works although no one really understands why (this is
especially called black magic).

3. n. [Stanford] A feature not
generally publicized that allows something otherwise impossible, or a
feature formerly in that category but now unveiled.

4. n. The ultimate goal of all
engineering & development, elegance in the extreme; from the first
corollary to Clarke's Third Law: “Any technology distinguishable from
magic is insufficiently advanced”.

Parodies playing on these senses of the term abound; some have made
their way into serious documentation, as when a MAGIC directive was
described in the Control Card Reference for GCOS c.1978. For more about
hackish ‘magic’, see Appendix
A. Compare black magic,
wizardly, deep magic,
heavy wizardry.